examples: Move discover_device function to utilities

While there are various functions which similar usage in all the examples,
I'd prefer to keep each example to be self-containing most of the things.
However some clearly repeated action can be moved to a single codebase.

examples/utilities.h

@@ -21,6 +21,7 @@
 #ifndef __UTILITIES_H
 #define __UTILITIES_H
 
+FpDevice * discover_device (GPtrArray *devices);
 FpFinger finger_chooser (void);
 const char * finger_to_string (FpFinger finger);